This invention relates in general to vehicular steering systems. In particular, this invention relates to control of vehicular electric power steering systems.
EPS systems typically rely on a column torque sensor to measure the driver's steering command inputs and apply an assistance torque to the rack, via the motor, in order to support the driver in steering the vehicle. In the rare case where the column torque sensor fails, there would be a sudden loss of the assistance provided by the motor, thus returning the system to a manual steering mode. Previous angle-based, limp-home assist functions have addressed torque sensor failures in two ways: either using the difference between the upper column position calculated by an external steering position and lower column position calculated by using the motor position or by using vehicle speed and motor position data from a lookup table. In the first instance, the difference between the lower column and upper column position can be used to estimate driver torque input to the steering system. This estimated driver torque can then be used to demand assistance using conventional torque based control. However, this method requires the presence of at least an external upper column position sensor. In absence of such external sensor, upper column position can be estimated using motor position that is compensated or otherwise correlated to steering torque required inputs through a gear compliance model. Modeling the gear compliance would typically require significant effort and face robustness challenges for accurate estimation of the driver torque input. In the second case, a lookup table can be tuned to estimate required assistance based on motor position and vehicle speed signals. The accuracy of this approach, however, can be compromised significantly when road conditions deviate from those used to generate the tuning maps and look up tables. Additionally, this methodology also does not typically take into account system non-linearity responses, such as gear friction.
Several challenges present themselves when implementing an angle based method, as described above. First, additional upper column position sensors may not be available in all systems. This may be due to a lack of available packaging space or costs issues associated with maintaining carry-over designs. Second, relying on a single, angle sensor based approach can result in a self steer condition, particularly when the tuned assistance becomes greater than actual rack force. A self steer condition exists where the vehicle continues to steer in one direction without any input from the driver. Such a situation can occur as a result of overestimating the operating rack force, resulting in an over assist condition. This is particularly evident on slippery or low “μ (‘mu’)” surfaces. Typically, rack forces are higher on a high μ road surface than when driving on a low μ surface. Third, when only a single sensor is used, bumps or pot holes generate transient impact force inputs that create control compensation issue. The steering wheel will respond to rack forces imparted by the bump and the resulting angle causes an unintended assist. In a normally operating EPS system, such road disturbances would be canceled because an opposing torque would have been instantly built up due to the steering inertia. Fourth, a vehicle driving in reverse also presents an issue when using angle based control. In reverse, the pneumatic trail of the tire is in front of the center of the tire patch. In the instance where the mechanical trail is less than this pneumatic trail, the aligning moment will be in the opposing direction.
Another issue arises when there is a quick/sudden reversal of steering direction relative to the initial direction of assist. This condition would be most evident in a counter steer movement to correct an over-steer condition. In this scenario, the driver would be cornering with a positive steer angle (CW) and column torque and therefore a positive assist applied to the rack. When a counter steer is requested, the driver attempts to turn the wheel CCW. This results in the driver applying a negative torque to the steering wheel while the steering position is still positive. In a functional EPS system, the motor assist would be in a negative direction, aiding the driver to turn the steering wheel in a CCW direction. However, in an angle-based control environment, the assist would remain positive until the angle becomes negative. This condition, while generating a reduced counter torque, still opposes the desired steering motion, which affects driver reaction. Furthermore, on low μ surfaces, the potential exists for the system/driver to over correct in a counter steer condition, making the vehicle harder to control.
Thus, an improved assist compensation method would be desirable.